Objectives To develop and test a method to assess adherence to a rotary diversified diet (RDD), a treatment for environmental illness, which is a putative disorder characterized by multiple sensitivities to foods, chemicals, or inhalants. The RDD requires the elimination of prohibited foods and rotation of remaining nonprohibited foods and their ''food families'' within a 4- to 7-day cycle. The regimen has yet to be validated to the satisfaction of the scientific community.Design Details of the 2 components of the RDD prescription, elimination and rotation, were documented, and a food record method of assessing adherence was developed. Adherence to the RDD was then assessed in a cohort of women who were enrolled in a larger prospective study. Test-retest reliability of the adherence assessment method was determined by calculating ratings twice on the same set of patient food records, with 1 week between trials.Subjects/setting All patients were contacted through a private environmental medicine clinic in Toronto, Canada. Eight patients provided the food records needed for development of the method; adherence was then assessed in 22 women aged 25 to 67 years.Statistical analyses Means, standard deviations, and 95% confidence intervals for adherence ratings were calculated. The reliability of the adherence assessment method was determined by calculating Pearson correlation coefficients for adherence ratings from each trial. A paired t test was also used to determine if the mean differences in ratings between trials were significant.Results Patients experienced difficulties following both components of the RDD: 37% to 44% of foods consumed were either prohibited or allowed, but were consumed on the incorrect day. The adherence assessment method was found to have high levels of reliability.Applications The adherence assessment method can be used in future evaluations of the RDD, although further testing of the method is recommended. Increased involvement of dietitians with patients diagnosed with environmental illness is recommended. J Am Diet Assoc. 1998;98:1439-1444.